The objective of the proposed studies is to use structural and biophysical methods to investigate the molecular mechanisms underlying the specific regulatory and targeting interactions of the Wnt signaling pathway. Wnt signaling plays an important role in embryonic development and in the regulation of cell growth. Inappropriate activation of Wnt signaling has been implicated in cancers and other human diseases. Dishevelled is an important player in the Wnt pathway. It relays the signal from the membrane-bound Wnt receptors to downstream partners. Sequence analysis revealed two new novel protein domains in Dishevelled: the DEP and DIX domains. It is believed that the DEP domain binds to the membrane-bound receptor and that the DIX domain interacts with downstream components. Protein NMR spectroscopy will be used to determine the structures of the two domains and the fundamental chemical nature of the interactions between both domains and their binding partners will be analyzed by structural and biophysical methods. Axin, one of the proteins downstream of Dishevelled, also contains a DIX domain. It has been hypothesized that the Wnt signal passes from Dishevelled to Axin through heterodimerization of the two DIX domains. This hypothesis will be examined by further analysis of the structures of the Axin DIX domain and the DIX heterodimer complex. The applicant suggests that these studies will reveal the first structural and functional information about DEP and DIX domains and, ultimately, may provide insights for the development of pharmaceutical agents that can interfere with specific Wnt signaling events in human disease.